The present invention relates to a polymer electrolyte fuel cell for use in portable power sources, distributed power sources, cogeneration systems and the like, and more particularly to improvements in its electrodes.
The fuel cell is generally constituted of unit cells comprising an electrolyte, a pair of electrodes which are disposed on both sides of the electrolyte and have gas diffusing and catalytic functions, an electrically conductive separator on the anode side that has a gas flow path to supply fuel like hydrogen to one electrode, and an electrically conductive separator on the cathode side that has a gas flow path to supply oxidant gas like air to the other electrode. And electricity and heat are simultaneously generated by electrochemically reacting the fuel and the oxidant.
In the polymer electrolyte fuel cell using a polymer electrolyte membrane as electrolyte to conduct hydrogen ions, a pair of catalyst layers, which are mainly composed of carbon powder with a platinum group catalyst carried thereon, are in closely attached to opposite side of the electrolyte membrane. Furthermore, a pair of electrode supporting materials which has both gas permeability and electric conductivity are placed closely on the respective outsides of the catalyst layers. The electrode supporting material and the catalyst layer constitute each electrode. Generally used electrode materials include carbon paper with a relatively high elasticity like paper, flexible carbon cloth and carbon felt.
Outside the electrodes are placed electrically conductive separator plates so as to mechanically clamp a membrane electrode assembly (MEA) and ca use the assembly to electrically connect with adjacent assemblies in series. A carbon plate is generally used as the separator. In the area where the separator plate and the electrode are in contact with each other, there is formed a gas flow path to supply the reaction gas and carry away generate gas or excessive gas.
In the electrode where hydrogen is supplied (anode), hydrogen that is supplied to the catalyst layer through the electrode supporting material from the gas flow path is oxidized into hydrogen ions. These hydrogen ions are transported to the electrode where air is supplied (cathode) by the electrolyte membrane. At the cathode, the hydrogen ions and oxygen react with each other to form water on the catalyst layer. The electrons that generate then flow from the anode to the cathode through an outside circuit. To have the process proceed efficiently, the electrode has to be provided with a suitable air passage to supply the reaction gas to the reaction area of the catalyst layer and efficiently remove drain gases such as water vapor formed in the catalytic reaction.
Hitherto, a polyfluorocarbon-type compound like polytetrafluoroethylene (PTFE) has been used as water repellent. That is, the polymer was mixed in the electrode supporting material or catalyst layer to secure the aforesaid air passage. To be concrete, carbon paper constituting the electrode supporting material is impregnated or coated with a colloid dispersion of a polyfluorocarbon-type compound and dried to remove the solvent, followed by heat treatment at 350 to 450xc2x0 C. to fix the polyfluorocarbon-type compound on the carbon paper. To impart water repellency to the catalyst layer, the following process has been adopted. A polyfluorocarbon-type compound repellent is fixed beforehand in carbon powder other than the platinum-carrying carbon powder. Then, the former carbon powder and the platinum-carrying carbon powder are evenly mixed. The polyfluorocarbon-type compounds that have been used include, in addition to PTFE, other polymers in which physical properties like glass transition point are changed by modifying various substituents like perfluoromethyl group.
In recent years, attempts have been made to control the water repellency of the component parts of the electrode thereby to efficiently discharge formed water by giving water repellency treatment to the electrode supporting material having gas permeability and electronic conductivity on the side that comes in contact with the catalyst layer and giving hydrophilic treatment with SiO2 to the other part.
In the method of giving water repellency treatment to the surface of electrically conductive cell component parts such as carbon particles, carbon fabrics or the like using a dispersion of a polyfluorocarbon-type compound like PTFE, it is difficult to coat the whole surface of the cell component parts with the polyfluorocarbon-type compound. Microscopically viewed, the component parts have sites not covered with the polyfluorocarbon-type compound. Those sites get wet in time, reducing the overall repellency. If the amount of the polyfluorocarbon-type compound applied is increased to raise the water repellency, that will deteriorate the electric contact between carbon fibers and carbon particles, reducing the cell performance.
It is pointed out that if the electrode supporting material is given a hydrophilic treatment with SiO2 or the like, the cell performance will drop probably because of the effect of the hydrophilic treatment agent. Electrode supporting materials merely subjected to such a hydrophilic treatment are low in water retention, and the cell performance is greatly influenced by changes in humidity of the supplied gas.
The polymer electrolyte fuel cell forms water in the electrodes during operation. The formed water evaporates and mixes with the supplied gas and discharged as drain gas. As a result, there arises a difference in humidity between the upstream side and the downstream side of the gas supply in a cell stack. This is more noticeable as the.utilization of gas increases. To raise the efficiency of the cell, an electrode is needed that is not so influenced by the humidity of the supplied gas.
In view of the prior art described above, including the disadvantages and deficiencies, it is an object of the present invention to provide a polymer electrolyte fuel cell that can maintain the cell performance at a high level.
The present invention provides a polymer electrolyte fuel cell comprising an anode, a cathode, a polymer electrolyte membrane interposed between the anode and the cathode, an anode-side separator plate having a gas flow path to supply fuel gas to the anode and a cathode-side separator plate having a gas flow path to supply oxidant gas to the cathode, wherein each of the anode and the cathode comprises a catalyst layer in contact with the polymer electrolyte membrane, an electrode supporting material having gas permeability and electronic conductivity, and a water repellent layer interposed between the catalyst layer and the electrode supporting material, the water repellent layer having through holes through which the catalyst layer and the electrode supporting material are electrically connected.
In a preferred mode of the present invention, the aforesaid through holes are distributed in such a manner that the average interval between the holes is smaller than the width of the gas flow path of the separator plate. That is, it is preferable that through the through holes distributed in such a state, the catalyst layer and the electrode supporting material are electrically connected with each other.
In another preferred mode of the present invention, the electrode supporting material is constituted of carbon fibers that have been subjected to a treatment for enlarging the surface area.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.